Recently, higher density magnetic recording is in high demand. As a technology for realizing high density of a magnetic recording, a perpendicular magnetic recording method is employed. The perpendicular magnetic recording medium based on this method includes at least a non-magnetic substrate and a magnetic recording layer including a hard magnetic material. The perpendicular magnetic recording medium may further optionally include, in addition to the elements, a soft magnetic underlayer which is formed from a soft magnetic material and plays a role for concentrating a magnetic flux generated by a magnetic head to the magnetic recording layer, an underlayer for orienting the hard magnetic material of the magnetic recording layer in an intended direction, a protective film for protecting a surface of the magnetic recording layer and the like.
As a material for forming a magnetic recording layer of the perpendicular magnetic recording medium, a granular magnetic material is used. This granular magnetic material is composed of a magnetic material and a non-magnetic material, and results in a granular structure composed of a magnetic crystal grain and precipitated non-magnetic material surrounding the periphery of the magnetic crystal grains. In the magnetic material, an ordered alloy, for example, has attracted attention in recent years, while in the non-magnetic material, carbon system, oxide system, nitride system and the like are known (see PTL 1 and the like, for example).
In the perpendicular magnetic recording medium, various materials are proposed for the granular magnetic material, but depending on a combination of the ordered alloy as the magnetic crystal grains and the non-magnetic material, if a film thickness of the magnetic recording layer increases, a phenomenon that the non-magnetic material precipitates not only on the grain boundary of the magnetic crystal grains but also on the surface of the magnetic crystal grains and interferes with growth of the magnetic crystal grains occurs in some cases (see NPL 1). If the film thickness of the magnetic recording layer further increases after such precipitation on the surface occurs, so-called secondary growth occurs that the magnetic crystal grains grow on the non-magnetic material having precipitated to the surface of the magnetic crystal grains. Such secondary growth results in lowering of magnetic anisotropy of the perpendicular magnetic recording medium.
Thus, various materials are examined also for the non-magnetic material contained in the granular magnetic material. For example, PTL 2 discloses a perpendicular magnetic recording medium containing a magnetic recording layer using B4C as the non-magnetic material. PTL 2 reports that high thermal stability, high magnetic anisotropy constant (Ku) and the like can be realized and a DC sputtering method enables the film formation. However, PTL 2 does not refer to use of B4C in combination with an ordered alloy.
PTL 3 discloses a thermal assist magnetic recording medium having a magnetic layer with a double structure using the granular magnetic material in which an oxide such as SiO2 is added as a non-magnetic material to an ordered alloy having an L10-type crystalline structure as a first magnetic layer and a continuous layer (CAP layer) not containing such oxide as a second magnetic layer. The invention of PTL 3 is characterized in that a content of the non-magnetic material in the first magnetic layer decreases from the substrate side toward the second magnetic layer side, and this constitution prevents precipitation of excessive non-magnetic material on an upper part of the crystal grains of the ordered alloy so as not to interrupt the growth of the crystal grains in a perpendicular direction. Moreover, as a result, crystal grains of the ordered alloy having a fine grain size and continuously grown in a direction perpendicular to a substrate surface are realized.